The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small-sized camera such as a camera for mounting in a portable device, which includes a cellular phone, a smartphone, and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones” have been more widely used, i.e., cellular phones with functions of portable information terminals (PDA) and/or personal computers. Since the smart phones generally are highly functional and exhibit high performance as opposed to the cellular phones, it is possible to directly edit images taken by a camera thereof without importing the images in personal computers or the like. In case of the smartphones, it is possible to use images taken by the cameras thereof in various applications. For example, the images may be used for creating characters of online games or makeup simulations. Such uses of the images, which were not conventionally common, are becoming increasingly popular every year with an increase in popularity of the smartphones.
Generally speaking, an imaging lens to be mounted in a portable device such as a cellular phone or a smartphone, which is developed for advanced users, is required to have optical performances that are equivalent to those of digital still cameras such as a high resolution and excellent color reproduction, as well as a small size due to limited space in a portable device. However, as the imaging lens to be mounted in a camera used for the above-described applications, it is not essential for the imaging lens for mounting in a camera to have the high resolution, but rather than that, it is more critical to have a wide angle of view, i.e., wide angle, in some applications.
An imaging lens having a three lens configuration can relatively satisfactorily correct aberrations and is also suitable for downsizing, so that such a lens has been widely used in the lens configuration of the imaging lens of the camera for mounting in the above-described portable devices. As the three lens configuration, there has been a well-known lens configuration, which includes a first lens having positive refractive power; a second lens having negative refractive power; and a third lens having positive or negative refractive power, arranged in this order from an object side. In the lens configuration, since the second lens has negative refractive power, it is relatively easy to achieve downsizing of the imaging lens. In addition, chromatic aberration generated in the first lens is suitably corrected through the second lens. Therefore, the lens configuration is one of effective lens configurations in view of downsizing of the imaging lens and satisfactory correction of aberrations.
However, in the lens configuration described above, since the second lens has negative refractive power, when it is attempted to attain both downsizing of the imaging lens and a wider angle, refractive power of each lens tends to be relatively strong. As a result, it is necessary to improve the fabrication accuracy and assembly accuracy of the lenses. For this reason, there has been recently proposed a lens configuration, in which refractive powers of all of the three lenses are positive. As the imaging lens having such three lens configuration, for example, an imaging lens described in Patent Reference 1 has been known.
According to the imaging lens disclosed in Patent Reference 1, the second lens and the third lens have inflexion points, and with aspheric surface shapes having the inflexion points, downsizing of the imaging lens and a wider angle are attained, while correcting aberrations.
In addition, although the lens configuration is slightly unfavorable in view of downsizing of the imaging lens and satisfactory correction of aberrations, as a lens that has a small F number and is bright, for example, there is known an imaging lens described in Patent Reference 2. Similar to the imaging lens disclosed in Patent Reference 1, in the imaging lens disclosed in Patent Reference 2, the first lens and the second lens have positive refractive powers. However, different from that in Patent Reference 1, the third lens has negative refractive power.    Patent Reference 1: Japanese Patent Application Publication No. 2012-014139    Patent Reference 2: Japanese Patent Application Publication No. 2010-145648
According to the imaging lens described in Patent Reference 1, it is possible to attain downsizing of the imaging lens while correcting aberrations to some extent. However, with increasing diversification in uses of images taken by cameras each year, imaging lenses are demanded to have even wider angles as one of added values of the imaging lens. Although the imaging lens described in Patent Reference 1 has the wider angle of view than that of conventional one, there is a limit by itself for attaining even wider angle.
Further, in case of the imaging lens described in Patent Reference 2, the whole lens system has a long focal length, and an axial length of the imaging lens is also long. Accordingly, it is difficult to achieve both wider angle and downsizing of the imaging lens.
Here, difficulty of attaining both downsizing and the wider angle while correcting aberrations satisfactorily is not a problem specific to the imaging lens to be mounted in portable devices such as the cellular phones and the smartphones. Rather, it is a common problem also for an imaging lens to be mounted in a relatively small camera such as digital still cameras, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain both downsizing and a wider angle, while satisfactorily correcting aberrations.
Further objects and advantages of the invention will be apparent from the following description of the invention.